Fix copy paste mistake (dotnet#5435)

Fix copy paste mistake and improve phrasing of RequestHeaderEncodingSelector and ResponseHeaderEncodingSelector.

Author: Suchiman

xml/System.Net.Http/SocketsHttpHandler.xml

@@ -724,8 +724,8 @@ For example, if the value is 64, then 65,536 bytes are allowed for the maximum r
         <ReturnType>System.Net.Http.HeaderEncodingSelector&lt;System.Net.Http.HttpRequestMessage&gt;</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets or sets a callback that decodes request headers values.</summary>
-        <value>The header encoding selector callback to decode the value for the specified response header name, or <see langword="null" /> to indicate the default behavior.</value>
+        <summary>Gets or sets a callback that selects the <see cref="System.Text.Encoding"/> to encode request header values.</summary>
+        <value>The header encoding selector callback that selects the <see cref="System.Text.Encoding"/> to encode the value for the specified request header name, or <see langword="null" /> to indicate the default behavior.</value>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -774,8 +774,8 @@ Draining occurs when a request is cancelled or a response is disposed prior to f
         <ReturnType>System.Net.Http.HeaderEncodingSelector&lt;System.Net.Http.HttpRequestMessage&gt;</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets or sets a callback that decodes response headers values.</summary>
-        <value>The header encoding selector callback to decode the value for the specified response header name, or <see langword="null" /> to indicate the default behavior.</value>
+        <summary>Gets or sets a callback that selects the <see cref="System.Text.Encoding"/> to decode response header values.</summary>
+        <value>The header encoding selector callback that selects the <see cref="System.Text.Encoding"/> to decode the value for the specified response header name, or <see langword="null" /> to indicate the default behavior.</value>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>